Self-assembled ionophores

ABSTRACT

Ionophores having the capacity spontaneously assemble in solution and composed of hydrogen bonded monomers of 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isoguanosine, 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-thio-isoguanosine or 2′,3′-Di-O-acetyl-5′-(t-butyl-dimethylsilyl)-isoguanosine are used to remove 137-cesium ions ( 137 Cs + ) from nuclear waste.

This application is a 371 of PCT/US98/04334, filed Mar. 11, 1998, and entitled to the right of priority of U.S. Provisional Application Serial No. 60/040,284 filed on Mar. 11, 1997, the entire contents of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ionophores which have the capacity to spontaneously assemble in solution. More particularly, the present invention concerns ionophore composed of hydrogen-bonded monomers of 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopyoidene-isoguanosine, or 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-thio-isoguanosine, and use of these ionophores for removing 137-cesium ions (¹³⁷Cs⁺) from nuclear waste.

2. Description of the Related Art

The United States faces a crisis in nuclear waste management. Safety and health issues surrounding nuclear waste have caused considerable interest in methods for separating radioactive isotopes^(4,5,9) Cesium-137 (¹³⁷Cs⁺) is a major product from uranium fission. Due to its 30 year half-life, much of the ¹³⁷Cs⁺ produced during the nuclear age still exists. ¹³⁷Cs⁺ and ⁹⁰Sr are the major heat sources in radioactive waste at Hanford Nuclear Reservation. Different methods for ¹³⁷Cs⁺ removal and/or recovery from nuclear waste have been described, including selective precipitation as phospho-tungstate salts, inorganic ion exchanges, and solvent extraction using crown ethers.^(4,5) Since the Na⁺ concentration in nuclear waste is 10³-10⁴ times that of ¹³⁷Cs⁺, ionophores with high Cs⁺/Na⁺ selectivity are required to separate ¹³⁷Cs⁺ from nuclear waste.

There are few Cs⁺-selective ionophores known. Due to its large size, selective extraction of Cs⁺ (r=1.67 Å) from solutions containing Na⁺ (r=0.97 Å) and K⁺ (r=1.33 Å) is challenging. Initially, design of Cs⁺ ionophores relied on large-ring crown ethers. Specifically, 21-c-7 and 23-c-8 have been proposed as ionophores for ¹³⁷Cs⁺ extractions.⁶ The use of large-ring crown ethers poses special challenges because of their flexibility. Thus, both the Cs⁺ binding constants and Cs⁺ selectivities of large-ring crown ethers are relatively modest.¹⁰ Better results have been obtained using rigid macrocycles, such as calixarenes.^(7,12-14) While their Cs⁺ binding constants and Cs⁺/K⁺ selectivities are often impressive, these rigid ionophores have two major problems: 1) they are difficult to synthesize and are available only in small quantities; and 2) due to high association constants, cation recovery from the ionophore complex is difficult.

SUMMARY OF THE INVENTION

The present invention solves the problems noted above by providing an ionophore useful for removing ¹³⁷Cs⁺ from nuclear waste. The term “ionophore” as used herein refers to molecule or an assembly of molecules which either has the capacity to bind one or more ions, or is actually bound to one or more ions. The ionophore of the present invention comprises a plurality of monomers, wherein each monomer is noncovalently bound to another monomer, preferably through hydrogen bonding. Ideally, the ionophore is capable of spontaneously assembling in a solution containing the plurality of monomers. These inophores self-assemble at concentrations as low as 1 μM in CHCl₃ or CH₃CN. The ionophore advantageously comprises identical monomers such as 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isoguanosine, 5′-(t-butyl-dimetlhylsilyl)-2′,3′-O-isopropylidene-thio-isoguanosine, or 2′,3′-Di-O-acetyl-5′-(t-butyl-dimethylsilyl)-isoguanosine. The ionophore may contain four or eight monomers. The ionophore is capable of forming a complex with an ion. This ion is preferably a cation such as Cs⁺, Na⁺, K⁺, Ag⁺, Hg⁺², Pb⁺², or Cd⁺², preferably Cs⁺, and most preferably ¹³⁷Cs⁺. Ideally, the binding affinity of the ionophore for Cs⁺ is greater than the binding affinity of the ionophore for Na⁺ or K⁺. The present invention includes the complex comprising the ionophore and the ion. Also, the present invention includes a micelle comprising a plurality of the ionophores, wherein each monomer comprises a hydrophobic moiety making micelle formation possible, wherein the ion is bound to one of the ionophores of the micelle; the hydrophobic moiety-containing monomer preferably has the following structure:

wherein R is H or —SiR″₃, where R″ is a hydrocarbon (CH₂)_(m) CH₃ wherein m=0-22, or an ester C=O(CH₂)_(n)CH₃, wherein n=0-22.

Additionally, the present invention is also directed to three methods:

(1) a method for forming the complex comprising the steps of adding the plurality of monomers to a solution containing the ion;

(2) a method for removing the ion from a solution comprising adding the plurality of monomers to the solution, and removing the resultant complex from the solution; and

(3) a method for removing the ion from an aqueous solution comprising adding a plurality of the hydophobic moiety-containing monomers to the solution. In method (3), the monomers form a composition comprising a micelle, wherein the micelle is composed of the ionophores, and the ionophores have ions bound thereto. The composition is then removed from the solution, preferably by ultrafiltration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a self-assembled ionophore, wherein hydrogen bonds form a cation-binding host.

FIG. 2 is a diagram illustrating that in the presence of certain cations, 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isoguanosine (isoG 1) forms an octamer, (isoG 1)₈-M⁺ 3.

FIG. 3 is a diagram illustrating a micellar ionophore formed by amphiphile 4.

FIG. 4 is a diagram illustrating that in the presence of certain cations, 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-thio-isoguanosine (thio-isoG 4) forms an octamer, (thio-isoG 4)₈-M⁺ 6.

FIG. 5 is a diagram illustrating the structures of 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isoguanosine (isoG 1), 2′,3′-Di-O-acetyl-5′-(t-butyl-dimethylsilyl)-isoguanosine (isoG 4), and 1,3-diisopropylcalix[4]arenecrown-6 5.

FIG. 6 illustrates a region of the 500 MHZ ¹H NMR spectra of a CDCl₃ solution of (isoG 1)₈-M⁺ 3 (2.0 mM) at 25° C. The top spectrum is of a sample formed by extraction of KI (2.5 M) from water. The middle spectrum is of a sample formed by extraction of CsI (0.005 M) from water. The bottom spectrum shows a sample after extraction from water containing KI (2.5 M) and CsI (0.005 M).

FIG. 7 is a diagram illustrating the synthesis of 2′,3′-Di-O-acetyl-5′-(t-butyl-dimethylsilyl)-isoguanosine (isoG 4).

FIG. 8 illustrates a 500 MHz ¹H NMR spectrum of 2′,3′-Di-O-acetyl-5′-(t-butyl-dimethylsilyl)-isoguanosine (isoG 4) (40 mM) in d₆-DMSO at 25° C. This spectrum indicates that isoG 4 is monomeric in d₆-DMSO.

FIG. 9 illustrates a region of the 500 MHz ¹H NMR specurum of 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isoguanosine (isoG 1) (16 mM) in CDCl₃ at 25° C. A) Before extraction; B) After extraction with water containing 5 mM cesium picrate. Integration of the picrate signal at a 8.78 ppm and resonances for isopropylidene 1 indicate formation of the octamer, (isoG 1)₈-Cs⁺ picrate.

FIG. 10 illustrates a region of the 500 MHz ¹H NMR spectrum of 2′,3′-Di-O-acetyl-5′-(t-butyl-dimethylsilyl)-isoguanosine (isoG 4) (16 mM) in CDCl₃ at 0° C. A) After extraction with water containing 4.5 mM cesium picrate; B) After extraction with water containing 4.5 mM potassium picrate; C) After extraction with water containing 4.5 mM potassium picrate and 4.5 mM cesium picrate. Integration of the diacetate's H8 resonance indicates a 57:43 ratio of (isoG 4)₈-K⁺ to (isoG 4)₈-Cs⁺.

FIG. 11 illustrates a 400 MHz ¹H NMR spectrum of 2′,3′-Di-O-acetyl-5′-(t-butyl-dimethylsilyl)-isoguanosine (isoG 4) (2.0 mM) in CD₃CN at 25° C. A) After titration with 0.50 mM (2 eq. per octamer) potassium picrate; B) After titration with-0.50 mM (2 eq. per octamer) cesium picrate; C) IsoG 4 (2.5 mM) with 0.31 mM (1 eq. per octamer) potassium picrate and 0.31 mM cesium picrate (1 eq. per octamer). The two sets of separate resonances for (isoG 4)₈K⁺ and (isoG 4)₈-Cs⁺ show there is little Cs⁺/K⁺ selectivity for isoG 4.

FIG. 12 illustrates an optical spectra of a CHCl₃ solution of 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isoguanosine (isoG 1) (0.11 mM) at 25° C.; A) Before cesium picrate extraction; B) After extraction of an aqueous cesium picrate (3.0 mM) solution. Integration of the isoG 1 absorption at 295 nm and the picrate absorption at 378 nm indicates formation of (isoG 1)₈-Cs⁺.

FIG. 13 illustrates a 65.6 MHz ¹³³Cs NMR spectra in CD₃CN at 25° C.; A) Cesium picrate (10 mM) before titrating of 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isoguanosine (isoG 1); B) After addition of 4 eq. of isoG 1. The limiting chemical shift change indicates that isoG 1 binds Cs⁺ as an octamer in CD₃CN. C) After addition of 8 eq. of isoG 1. D) After addition of 16 eq. of isoG 1.

FIG. 14 illustrates a stack plot of spectra from ¹³³Cs inversion-recovery T₁ experiments in CD₃CN at 20° C.; A) Cesium picrate (10 mM). Based on the null point determination, the T₁ is 3.25 s; B) For (IsoG 1)₈-Cs⁺ (8 mM). The T₁ is 0.0023 s.

FIG. 15 illustrates a 65.6 MHz ¹³³Cs NMR spectra in CDCl₃ at 0° C.; A) For (isoG 4)₈-Cs⁺ (2.0 mM); B) For calixarene 5 (2.0 mM); C) for a 1:1 mixture of (isoG 4)₈-Cs⁺ and calixarene 5 (both 2.0 mM). Integration of the two signals gives a 1:1 ratio. This experiment shows that (isoG 4)₈ and calixarene 5 have similar Cs⁺ association constants.

FIG. 16 illustrates a region of the 500 MHz ¹H NMR spectrum for solutions of (isoG 1)₈-Cs⁺ and calixarene 5 in CDCl₃ at 0° C.; A) For a solution containing 2 mM (isoG 1)B-Cs⁺; B) For a solution containing 1.7 mM (isoG 1)₈-Cs⁺ and 17 mM calixarenie 5. Even in the presence of 10 equivalents of calixarene 5 (log K_(a)(Cs⁺)=8.8) the NH1 peak for (isoG 1)₈-Cs⁺ octamer predominates.

FIG. 17 illustrates a 65.6 MHz ¹³³Cs NMR spectra in CDCl₃ at 0° C.; A) For calixarene 5-Cs⁺ complex (2.0 mM); B) For (isoG 1)₈-Cs⁺ (2.0) mM); C) For a solution containing a 1:1 mixture of (isoG 1)₈-Cs⁺ (2.0 mM) and calixarene-Cs⁺ 5 (2.0 mM); D) For a solution containing a 1:10 mixture of (isoG 1-Cs⁺ (1.7 mM) and calixarene 5 (17 mM). The major ¹³³Cs resonances corresponds to that for (isoG 1)₈-Cs⁺. There is no evidence for a calixarene-Cs⁺ peak. There is a small amount of an unknown complex at σ=−58.8 ppm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention involves a new approach for ion complexation. Ion binding and transport are fundamental to many chemical and biological processes, and thousands of natural and synthetic ionophores are known. Usually, covalent bonds constrain a rigid host into a “preorganized” conformation for productive binding. The novelty of the present invention is in the design of ionophores which self-assemble through noncovalent interactions (see FIG. 1). This alternative ionophore design uses hydrogen bonds to build self-assembled structures that coordinate ions.¹⁵⁻¹⁷ Cation-binding affinity and selectivity may be achieved through cooperative assembly of the host. 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isoguanosine (isoG 1) is the focus of the present invention. IsoG 1 self-associates in organic solvents to form a stable tetramer, (isoG 1)₄ 2 (see FIG. 2).^(17,18) The tetramer (isoG 1)₄ 2, with four oxygens in its central cavity, has a high affinity for cations. IsoG 1 coordinates K⁺ to form octomer (isoG 1)₈-K⁺ 3, with a binding constant rivaling that of 18-c-6 derivatives.^(17b) It is believed that the isopropylidene's conformational rigidity facilitates self-association of isoG 1. The sugar of isoG 1 influences both the Cs⁺ affinity and Cs⁺/K⁺ selectivity of the self-assembled ionophore. Specifically, isoG 1 forms a self-assembled ionophore with remarkable Cs⁺ selectivity.

One embodiment of the present invention is the use of self-assembled ionophores for environmental applications (e.g., the effective removal of ¹³⁷Cs⁺ from nuclear waste). One of the goals of the present invention, therefore, is to make Cs⁺ selective self-assembled ionophores. Through an understanding of structural factors that control host association and cation binding, additional self-assembled ionophores that selectively bind and transport cations are developed, such as micellar ionophores for practical Cs⁺ separations. Organic synthesis is used to prepare a series of structural analogs. Various techniques, including X-ray crystallography, electrospray mass spectrometry, and multinuclear NMR spectroscopy are used to determine the structure of the self-assembled ionophores. The self-assembly and cation binding thermodynamics for these isoG 1 analogs is determined using titration calorimetry. Correlation of thermodynamics with structure facilitates design of better self-assembled ionophores. Self-assembled ionophores are used for metal purification, as new materials, and as artificial ion channels.

In the inventive ionophores, self-association via hydrogen bonds results in the formation of a cavity capable of binding cations with high affinity and selectivity. A self-assembled ionophore has two major advantages over covalent ionophores: ease in synthesis, and ease in removal of the cation from the ionophore complex, which should be readily dismantled, allowing the cation and monomer to be separated and recycled.

Certain nucleosides have hydrogen-bonding donor and acceptor groups that allow them to self-associate. IsoG 1 self-assembles to form the tetramer (isoG 1)₄ 2 in organic solvents.¹⁻² This tetramer, formed by hydrogen bonds, has four nucleophilic oxygen atoms pointing into its central cavity. These oxygen atoms are able to coordinate metal cations. In the presence of certain cations, IsoG 1 forms a remarkably stable octamer, (isoG 1)₈-M⁺ 3 (see FIG. 2). In particular, isoG 1 has an unusually strong affinity and selectivity for Cs⁺ versus K⁺ and Na⁺.³ The Cs⁺ binding constants rival those of covalent ionophores, such as crown ethers and cryptands.

The self-assembled ionophore (isoG 1)₈-M⁺ 3 coordinates metal cations in homogeneous organic solution, and also removes cations from water in liquid-liquid extractions. To overcome the obvious limitations of liquid-liquid extractions, advantage is taken of the unique self-assembly and ion binding properties of isoG 1 to form micelles that will bind Cs⁺ in water. One potential precursor of these micelles is amphiphile 4, depicted in FIG. 3. By attaching a long-chain alkyl group to the 5′-position of isoG 1, micelles are formed in aqueous solution. Micelles “studded” with the isoG 1 head group form tetramers at the lipid-water interface, and thus coordinate metal cations. Electrostatic interactions at the micellar surface in water are intermediate in strength when compared to the same interactions in water and in the gas phase.⁸ Similarly, isoG 1 self-assembly in the micelle is strong enough that coordination of metal cations at the membrane surface is possible. Again, initial experiments focus on the coordination of Cs⁺ from aqueous solution. As depicted in FIG. 3, Cs⁺ binding is assayed using an ultrafiltration membrane. Thus, an aqueous solution of Cs⁺ salts and the micelle result in an equilibrium mixture of salt bound to the micelle. Since the molecular weight of the micelle is large, filtration through a membrane allows only small molecules, such as unbound Cs⁺ and monomeric amphiphile 4, to pass through the filter. In contrast, any Cs⁺ bound to the micelle does not pass through the membrane. Measurement of Cs⁺ concentration, before and after addition of the micelle allows calculation of an equilibrium constant for Cs⁺ binding. From a practical point of view, the ultrafiltration strategy for metal purification has obvious advantages over liquid-liquid extractions.

Ultimately, self-assembled ionophores with different cavity sizes are made. By varying the cavity size, the ionophore's cation selectivity is changed. Variation of the cavity's hydrogen-bond donors and acceptors is one strategy for tailoring ion selectivity. Models indicate that changing the C2 substituent of isoG 1 from O to S, to give 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-thio-isoguanosine (thio-isoG 4) (see FIG. 4), reduces the tetramer cavity diameter from 2.8-3.0 Å in 2 to 2.1-2.3 Å in tetramer (thio-isoG 4)₄ 5. This change favors binding cations with 1.0-1.2 Å radii, rather than the larger cations (1.3-1.5 Å) that coordinate best to the tetramer (isoG 1)₄ 2. Also, thiophilic metals will have a higher affinity for tetramer (thio-isoG 4)₄ 5 relative to the oxo-containing tetramer (isoG 1)₄ 2.

The self-assembly and metal-binding properties of thio-isoG 4 is evaluated. Thiophilic cations organize thio-isoG 4 into higher-ordered structures such as tetramer (thio-isoG 4)₄ 5 or octamer (thio-isoG 4)₈ 6 (see FIG. 4). Environmentally important cations like Ag⁺, Hg⁺², Pb⁺², and Cd⁺², all which bind well to sulfur ligands, are particuiarly addressed by these ionophores.²⁴

The present invention is illustrated more specifically by referring to the following Example. However, nothing in this example shall be taken as a limitation upon the overall scope of the invention.

EXAMPLE

The self-association and Cs⁺/K⁺ binding properties of isopropylidene 1 and 2′,3′-O-diacetyl isoG 4 were compared (isoG 1 binds K⁺ over Na⁺ by at least 10-fold) (see FIG. 5). The different propensity for 1 and 4 to self-associate in organic solvents was apparent when comparing ¹H NMR spectra. In the absence of metal ion, isopropylidene 1 forms a hydrogen bonded tetramer 2 in CD₃CN (Ka=10⁹ M-³),^(17a) while diacetate 4 is monomeric under identical conditions. These results are consistent with the proposal that nucleobase-sugar hydrogen bonds drive the self-association of isopropylidene 1.

Both isopropylidene 1 and diacetate 4 coordinate K⁺ and Cs⁺ strongly in CDCl₃ and CD₃CN. Integration of ¹H NMR and UV-VIS spectra after metal picrate extraction from water into CDCl₃ indicate that 1 and 4 bind K⁺ and Cs⁺ to form (isoG)₈-M⁺ 3 (metal picrate titrations also showed that isoG 1 and 4 form octanes, (isoG)-M⁺ 3 in CD₃CN). Since both cesium and potassium picrate are insoluble in CDCl₃, spectroscopic measurement of the picrate anion is indirect evidence for cation binding by 1 and 4. Cesium-133 NMR directly showed that these isoG analogs bind Cs⁺.¹⁹ Distinct ¹³³Cs NMR spectra were obtained after cesium picrate extraction by isopropylidene 1 (σ −55.2 ppm) and by diacetate 4 (σ −28.4 ppm) (the ¹³³CS chemical shifts are relative to 0.5 M CsI in D₂O at 0° C.). The unique ¹³³Cs chemical shifts indicate that the electronic environment around Cs⁺ is different in the two (isoG)₈-Cs⁺ species.

Cation binding was also indicated by a decrease in the ¹³³Cs T₁ value in the presence of isopropylidene 1. Typically, ¹³³Cs T₁ values decrease upon complexation by ionophores, since ¹³³Cs relaxation is dominated by its nuclear quadrupole and by its reorientational correlation time, t_(c).^(20.21) First, coordination and desolvation change the electric field gradient near Cs⁺. Second, t_(c) for an ionophore-metal complex is larger than that for a solvated Cs⁺. The ¹³³Cs T₁ values in CD₃CN were 3.25 s for cesium picrate and 0.0023 s for (isoG 1)₈-Cs⁺. This 1400-fold decrease in ¹³³Cs T, is then consistent with Cs⁺ coordination by (isoG 1)₈.

Cesium binding constants (K_(a)) for 1 and 4 in CDCl₃ were determined from NMR competition experiments with 1,3-diisopropylcalix[4]arenecrown-6 5. Calixarene 5 is a Cs⁺ selective ionophore, with log K_(a) (Cs⁺)=8.8 in CDCl₃ (Table 1).^(13a) Coordination of Cs⁺ by calixarene 5 can be monitored by both ¹H and ¹³³Cs NMR, since the free and Cs⁺ bound calixarene 5 are in slow exchange. Addition of one equivalent of calixarene 5 to a CDCl₃ solution of (isog 4)₈-Cs⁺ gave two separate ¹³³Cs NMR signals in a 1.0:1.0 ratio, with one resonance for (isoG 4)₈-Cs⁺ at −28.6 ppm, and one for Cs⁺-bound calixarene 5 at −61.4 ppm. The diacetate octamer, (isoG 4)₈, with a Cs⁺ binding constant equal to that of calixarene 5, is a potent ionophore (Table 1). Competition experiments showed that isopropylidene 1 binds Cs⁺ even more strongly than does calixarene 5. Upon addition of ten equivalents of calixarene 5 to a CDCl3 solution containing (isoG 1)₈-Cs⁺ there were no changes in the ¹H and ¹³³Cs NMR spectra. This experiment establishes a lower limit of log K_(a) (Cs⁺)=9.8 for isopropylidene 1. Calixarene 5 did not remove Cs⁺ from the isopropylidene octamer, (isoG 1)₈-Cs⁺, as it did in competition experiments with diacetate (isoG 4)₈-Cs⁺. The Cs⁺ binding constant for isopropylidene 1 is clearly greater than that for diacetate 4, indicating that isoG's sugar group influences the ionophore-cation interaction.

Binding affinity is only one measure of an ionophore's utility. An effective ionophore should also be ion selective. ¹H NMR spectroscopy was used to determine the Cs⁺/K⁺ binding selectivities for diacetate 4 and isopropylidene 1. Since (isoG)₈-K⁺ and (isoG)₈-Cs⁺ were in slow exchange on the NMR time-scale for both 1 and 4, Cs⁺/K⁺ selectivities could be determined by integrating NMR signals for the separate (isoG)-M⁺ species. Diacetate 4 had little Cs⁺/K⁺ selectivity. Extraction of water containing equimolar concentrations (4.5 mM) of potassium picrate and cesium picrate with a CDCl₃ solution of diacetate 4 (16 mM) gave 53% (isoG 4)-K⁺ and 47% (isoG 4)₈-Cs⁺, for a Cs⁺/K⁺ selectivity of 0.89. The free energy difference for coordination of Cs⁺ vs K⁺ in CDCl₃ by 4 is small (ΔΔG<0.05 kcal/mol) (a similar Cs⁺/K⁺ selectivity factor of 0.83 ((ΔΔG=0.11 kcal/mol) in CD₃CN was determined from titration of (isoG 4)₈-Cs⁺ with potassium picrate).

In contrast to the indiscriminate diacetate 4, isopropylidene 1 is a Cs⁺ selective ionophore. When a CDCl₃ solution of isopropylidene 1 was stirred with water containing equimolar potassium picrate and cesium picrate, only Cs⁺ was extracted into the organic phase. To observe any (isoG 1)8-K⁺ complex, the K⁺/Cs⁺ ratio had to be increased. Extraction of water containing 2.50 M KI and 0.005 M CsI with a CDCl₃ solution of 1 gave (isoG 1)₈-K⁺ (σ 14.03 for NH1) and (isoG 1)₈-Cs⁺ (σ 13.53 for NH1) in a 1.5:1 ratio (FIG. 6). Thus, isopropylidene 1 has a Cs⁺/K⁺ binding selectivity of approximately 333/1, corresponding to a relative free energy that is 3.5 kcal/mol more favorable for Cs⁺ binding (see Table 1).

TABLE 1 Cs⁺Binding Constants and Cs⁺/K⁺Selectivities for Various Ionophores. Ionophore log K_(a) (Cs⁺) K_(a) (Cs⁺)/K_(a) (K⁺) Ref. Calix[4]crown 5 8.8 250 13a^(a) (IsoG 4)₈ 8.8 0.86 this application^(b) (IsoG 1)₈ >9.8 333 this application ^(a)In CHCl₃ saturated with H₂O at 22° C. ^(b)In CDCl₃ saturated with H₂O at 20° C.

Thus, it has been shown that isoG's sugar substituents influences the self-assembled ionophore's cation selectivity. Compared with diacetate 4, both the absolute Cs⁺ binding constant and the Cs⁺/K⁺ selectivity are significantly greater for isopropylidene 1. This change in the ribose's 2′,3′-substitution alters the Cs⁺/K⁺ selectivity ratio by 400-fold. Isopropylidene 1 likely forms such an effective self-assembled ionophore due to “preorganization” on two different levels.²⁰ First, the 2′,3′-isopropylidene constrains the sugar conformation so as to optimize hydrogen bonds that stabilize the tetramer, (isoG 1)₄. Once self-assembled, the tetramer's four carbonyl oxygens are then well oriented to coordinate cations.^(11b) From a practical viewpoint, self-association of isopropylidene 1 demonstrates the ability to bind metal cations with high affinity and selectivity.

EXPERIMENTAL SECTION

Synthesis.

Isopropylidene 1 was prepared as described.¹ Isopropylidene 1 was purified by flash chromatography on silica gel using 10:1 CH₂Cl₂:EtOH as eluant. N⁶-Formamidine isoG (a in FIG. 7) was synthesized as described.²⁵

5′-(tert-Butylldimethylsilyl)-N⁶-formamidine isoG (b in FIG. 7).

To a solution of N⁶-formamidine isoG a (0.52 g, 1.53 mmol) in DMF (15 mL) was added NEt₃ (0.61 g. 0.84 mL, 6.03 mmol) and TBDMS-Cl (0.88 g, 5.85 mmol). The reaction mixture was stirred at rt for 48 h, diluted with CH₂Cl₂ (30 mL) and the organic layer was washed with H₂O (15 mL), 0.01 N HCl (15 mL), saturated NaHCO₃ (15 mL), and saturated NaCl (15 mL). The organic layer was dried over Na₂SO₄ and concentrated in vacuo to give a yellow gum. The crude product was triturated with diethyl ether to give b (0.57 g, 83 %) as a white solid. R_(ƒ)0.75 (CH₂Cl₂/MeOH, 3:2). UV (MeOH), ^(λ)max (ε 227 (24000), 260 (14000), 345 (25000). ¹H NMR (400 MHz, DMSO-d₆) δ 0.04 (s, 6 H), 0.86 (s, 9 H0, 3.09 (s, 3 H), 3.19 (s, 3 H), 3.71 (dd, J=4.4, 11.3 Hz, 1 H), 3.81 (dd, J=3.8, 11.3 Hz, 1 H), 3.89 (ddd, J=3.8, 4.1, 4.4 Hz, 1 H), 4.07 (ddd, J=4.1, 5.0, 5.4 Hz, 1 H), 4.38 (ddd, J=5.0, 5.2, 5.9 Hz, 1 H), 5.18 (d, J=5.4 Hz, 1 H), 5.51 (d, J=5.9 Hz, 1 H), 5.69 (d, J=5.2 Hz, 1 H), 7.97 (s, 1 H), 9.11 (s, 1 H), 11.05 (br s, 1 H). ¹³C NMR (50 MHz, DMSO-d₆) δ −5.5, 18.0, 25.8, 34.3, 63.0, 69.9, 73.2, 84.3, 86.3, 113.1, 139.3, 154.4, 156.4, 157.7, 161.0. LRMS (FAB), m/z (rel int) 73 (100), 207 (88), 453 ([M+1]⁺, 33). HRMS (FAB), calc. For C₁₉H₃₃N₆O₅Si 453.2282, found 453.2285.

5′-(tert-Butyldimethylsilyl)-isoguanosine (c in FIG. 7).

To a solution of 5′-(tert-butyldimethylsilyl)-N⁶-formamidine isoG b (0.30 g, 0.66 mmol) in CH₃CN (2 mL) was added EtOH (2 mL) and NH₄OH (6 mL). The reaction mixture was stirred at rt for 16 h after which time TLC indicated the reaction was complete. The solvent was evaporated to give c (0.26 g, 100%) as a white solid. This material was used without further purification in the next step. Rƒ0.58 (CH₂Cl₂/MeOH, 3:2). ¹H NMR (200 MHz, DMSO-d₆) δ 0.05 (s, 6 H), 0.87 (s, 9 H), 3.66-3.90 (m, 3 H), 4.08 (m, 1 H), 4.31 (dd, J=4.7, 4.7 Hz, 1 H), 5.14 (br s, 1 H), 5.22 (br s, 1 H), 5.66 (d, J=4.7 Hz, 1 H), 7.46 (br s, 1 H), 7.90 (s, 1 H). LRMS (FAB), m/z (rel int) 73 (100), 75 (27), 152 (69), 398 ([M+1]^(+, 18)). HRMS (FAB), calc. For C₁₆H₂₈N₅O₅Si 398.1860, found 398.1842.

2′,3′-Di-O-acetyl-5′-(tert-Butyldimethylsilyl)-isoguanosine (4 in FIG. 7).

To a suspension of 5′-(tert-butyldimethylsilyl)-isoG c (0.26 g, 0.66 mmol) in CH₂Cl₂ was added NEt₃ (0.67 g, 0.93 mL, 6.63 mmol) and acetic anhydride (0.68 g, 0.63 mL, 6.63 mmol). The reaction was allowed to stir at rt for 19 h after which time TLC indicated that the reaction was complete. The reaction was diluted with CH₂Cl₂ (25 mL) and the organic layer was washed with 0.01 N HCl (25 mL), sat NaHCO₃ (25 mL), and sat NaCl (25 mL). The organic layer was dried over Na₂SO₄ and concentrated in vacuo to give a yellow oil. The ¹H NMR of the crude product showed many acetate signals. Thus, MeOH (10 mL) and CH₂Cl₂ (5 mL) were added and the reaction mixture was refluxed for 72 h. After this time the two spots with Rƒ0.69 and 0.53 (CH₂Cl₂/MeOH, 10: 1) were converted to two lower running spots of Rƒ0.41 and 0.27 (major). This mixture was purified by silica gel chromatography to give 2′,3′-di-O-acetyl-5′-(tert-butyldimethylsilyl)-isoG 4 (0.11 g, 34%) as a white solid. Rƒ0.27 (CH₂Cl₂/MeOH, 10:1). UV (MeOH), ^(λ)max (ε) 215 (32000), 252 (12000), 295 (12000). ¹H NMR (400 MHz, DMSO-d₆) δ 0.07 (s, 6 H), 0.88 (s, 9 H), 2.00 (s, 3 H), 2.11 (s, 3 H), 3.81 (dd, J=3.5, 11.3 Hz, 1 H), 3.88 (dd, J=3.5, 11.3 Hz, 1 H), 4.20 (m, 1 H), 5.41 (dd, J=2.9, 5.2 Hz, 1 H), 5.70 (dd, J=5.2, 6.6 Hz, 1 H), 5.96 (d, J=6.6 Hz, 1 H), 7.62 (br s, 1 H), 7.91 (s, 1 H), 10.51 (br s, 1 H). ¹³C NMR (50 MHz, CDCl₃) δ −5.5, 18.4, 20.4, 20.7, 25.9, 63.1, 71.8, 74.2, 83.7, 83.7, 110.0, 136.5, 151.5, 154.5, 158.0, 169.4, 170.0. LRMS (FAB), m/z (rel int) 66 (29), 73 (100), 152 (40), 482 ([M+1]⁺, 38). HRMS (FAB), cailc. for C₂₀H₃₂N₅O₇Si 482.2071, found 482.2056.

Preparation of Potassium and Cesium Picrate:

Potassium picrate was prepared by neutralizing picric acid with an equal molar amount of potassium or cesium hydroxide in EtOH. Solid impurities were removed by filtration. The resulting metal picrate salts, which precipitated from solution, were recrystallized twice from water and dried in a vacuum desiccator for 2 days. The recrystallized metal picrates salts were characterized by ¹H NMR, UV spectroscopy, and elemental analysis.

NMR Experiments

Most NMR experiments were performed on a Bruker AMX-500 NMR spectrometer. The spectrometer ¹H frequency was 500.13 MHz and its ¹³³Cs frequency was 65.6 MHz. The temperature was controlled to ±0.1° C. The spectral window was 20 ppm for ¹H and 300 ppm for ¹³³Cs. Typical 90° pulse widths were 11 μs for ¹H and 7.4 μs for ¹³³Cs. The 133Cs chemical shifts were referenced relative to 0.5 M CsI in D₂O at 0° C.

Metal Cation Picrate Extractions:

To a glass vial containing 1 mL of a 16 mM solution of isoG 1 or 4 in CDCl₃ was added 1 mL of a 4.5 mM solution of metal picrate in distilled water. This CDCl₃/H₂O mixture was stirred for 1 h at rt. The mixture was then transferred to an eppendorf tube and centrifuged for 5 min to force the organic layer to the bottom of the tube. An aliquot (0.5 mL) of the CDCl₃ layer was carefully removed with a syringe and transferred to an NMR tube for measurement. The 500 MHz ¹H NMR spectrum of the sample was recorded at 0° C. The stoichiometry for (isoG)₈-M⁺ was determined by comparing the integration of the picrate's ¹H resonance at 8.66 ppm with the integration of the isoG resonances.

Competition Experiment for Potassium Picrate and Cesium Picrate Extraction.

To a glass vial containing 1 mL of a 16 mM solution of isoG 4 in CDCl₃ was added a 1 mL solution of 4.5 mM potassium picrate and 4.5 mM cesium picrate in H₂O. This CDCl₃/H₂O mixture was stirred for 1 h at rt. The mixture was then transferred to an eppendorf tube and centrifuged for 5 min to force the organic layer to the bottom of the tube. An aliquot (0.5 mL) of the CDCl₃ layer was carefully removed with a syringe and transferred to an NMR tube for measurement. The ¹H NMR (500 MHz) spectrum was recorded at 0° C. The relative amounts of (isoG)₈-K⁺ and (isoG)₈-Cs⁺ were determined by comparing the integration of the H8 resonance of (isoG)₈-K⁺ at 7.89 ppm, with the integration of the H8 resonance of (isoG)₈-Cs⁺ at 7.68 ppm.

Metal Picrate Titrations.

A series of NMR tubes containing a solution of isoG 4 (2.6 mM) in CD₃CN, and metal cation picrate in CD₃CN in increasing increments (0.02 mM, 0.04 mM, 0.08 mM, 0.16 mM, 0.32 mM) were vortexed for 2 min. The 400 MHz ¹H NMR spectra were recorded at 25° C. The stoichiometry of the isoG-metal complex was determined by comparing the integration of the picrate's ¹H resonance at 8.66 ppm with the integration of the isoG resonances.

¹³³Cs T₁ Determination:

The inversion-recovery method was used to determine the ¹³³Cs spin lattice relaxation time, T₁ for cesium picrate (10 mM) and (isoG 1)₈-Cs⁺ picrate (8 mM) in CD₃CN at 25° C. The T₁ values were estimated by a null point determination method in a single inversion recovery experiment. Incremental values for τ of 0.05 sec and 0.002 sec were used for cesium picrate and (isoG 1)₈-Cs⁺ picrate, respectively, to determine the null point. Selected acquisition parameters of the individual spectra were as follows: spectral window, 200 ppm; relaxation delay 10 S; ¹³³Cs 90 degree pulse 7.3 us; number of scans 300.

Competition of (isoG 4)₈-Cs⁺ with Calixarene 5.

To a glass vial containing 1 mL of a 16 mM solution of isoG 4 in CDCl₃ was added 1 mL of a 4.5 mM solution of cesium picrate in distilled water. This CDCl₃/H₂O mixture was stirred for 1 h at rt. The mixture was then transferred to an eppendorf tube and centrifuged for 5 min to force the organic layer to the bottom of the tube. An aliquot (0.5 mL) of the CDCl₃ layer was carefully removed with a syringe and transferred to an NMR tube for measurement. To this was added 50 mL of a 20 mM solution of calixarene 5, so that the overall solution was 1.8 mM4 (isoG 4)₈-Cs⁺ and 1.8 mM calixarene 5. The ¹H NMR (500 MHz) and ¹³³Cs NMR (65.6 MHz) spectra were recorded at 0° C. The relative amounts of (isoG 4)₈-Cs⁺ and (isoG 4)₄ were determined by comparing the integration of ¹³³Cs resonance for (isoG)₈-Cs⁺ at −28.4 ppm with the integration of the calixarene-Cs⁺ resonance at −61.4 ppm.

Competition of (isoG 1)₈-Cs⁺ with Calixarene 5.

To a glass vial contain 1 mL of a 16 mM solution of isoG 1 in CDCl₃ was added 1 mL of a 4.5 mM solution of cesium picrate in distilled water. This CDCl₃/H₂O mixture was stirred for 1 h at rt. The mixture was then transferred to an eppendorf tube and centrifuged for 5 min to force the organic layer to the bottom of the tube. An aliquot (0.5 mL) of the CDCl₃ layer was carefully removed with a syringe and transferred to an NMR tube for measurement. To this was added varying amounts of a 20 mM solution of calixarene 5, so that the overall solution was 1.7 mM (isoG 1)₈-Cs⁺ and between 1.7-17 mM in calixarene 5. The ¹H NMR (500 MHz) and ¹³³Cs NMR (65.6 MHz) spectra-were recorded at 0° C.

Determination of Cs⁺/K⁺ Specificity for Isopropylidene isoG 1.

Because of solubility problems in obtaining high concentrations of potassium picrate, the more soluble iodide salts were used to determine the Cs⁺/K⁺ specificity for isopropylidene 1. To a glass vial containing 1 mL of a 16 mM solution of isoG 1 in CDCl₃ was added a 1 mL solution of 2.5 M KI and 0.005 M CsI in H₂O. This CDCl₃/H₂O mixture was stirred for 1 h at rt. The mixture was then transferred to an eppendorf tube and centrifuiged for 5 min to force the organic layer to the bottom of the tube. An aliquot (0.5 nL) of the CDCl₃ layer was carefully removed with a syringe and transferred to an NMR tube for measurement. The ¹H NMR (500 MHz) spectrum was recorded at 25° C. The Cs⁺/K⁺ specificity was determined by peak integration of the separate K⁺ and Cs⁺-bound species.

TABLE 2 ¹H NMR chemical shifts (ppm) for isoG diacetate 4. Solvent Resonance DMSO-d_(b) ^(a) CDCl_(a) ^(b) CDCl_(a)+K^(30 c) CDCl_(a)+Cs^(30 d) CD_(a)CN+K^(30 e) CD_(a)CN+Cs^(30 f) NH1 10.51 14.21 14.24 14.24 14.02 NH6_(A) 7.62 10.95 10.95 10.84 10.90 NH6_(B) 7.62 6.53 6.74 6.19 6.31 H8 7.91 7.86 7.89 7.68 7.98 7.84 H1¹ 5.96 6.14 5.74 5.86 5.73 5.63 H2¹ 5.70 5.55 5.40 5.38 5.38 5.37 H3¹ 5.41 5.42 5.40 5.38 5.33 5.24 H4¹ 4.20 4.25 4.33 4.37 4.33 4.33 H5^(f) 3.88 3.90 4.01 4.04 4.09 4.11 H5^(f) 3.81 3.86 3.88 3.93 3.91 3.88 CH₁ A 211 2.15 2.15 2.17 2.15 2.11 CH₁ B 2.00 2.02 2.01 24 2.08 2.02 tBu 0.88 0.93 0.93 0.94 0.95 0.94 SiMe A 0.07 0.14 0.15 0.10 0.18 0.17 SiMe B 0.07 0.14 0.15 0.16 0.15 0.14 ^(a)400 MHz at 25° C. ^(b)500 MHz at 20° C. ^(c)500 MHz at 0° C. after extraction of potassium picrate. ^(d)500 MHz at 0° C. after extraction of cesium picrate. ^(e)400 MHz at 25° C. with excess potassium picrate. ^(f)400 MHz at 25° C. with excess cesium picrate.

TABLE 3 ¹³³Cs NMR chemical shifts^(a) Species α(ppm) (IsoG 1)₈-Cs⁺ −28.6 (IsoG 4)₈-Cs⁺ −55.1 calixarene 5-Cs⁺ −61.4 ^(a)At 65.6 MHz in CDCl₃ at 0° C. Relative to KI in D₂O at 0° C.

REFERENCES

The following publications are hereby incorporated by reference.

1. “Self-Assembled Ionophores from Isoguanosine.” Davis, J. T.; Tirumala, S.; Jenssen, J. R.; Radler, E.; Fabris, D., J. Org. Chem. 1995, 60, 4167-4176.

2. “Self-Assembled Ionophores. An Isoguanosine-K+ Octamer.” Tirumala, S.; Davis, J. T., J. Am. Chem. Soc. 1997, accepted with revisions.

3. “The Role of Ribose Conformation in Cesium Selective Self-Assembled Ionophores from Isoguanosine.” Tirumala, S.; Marlow, A. L.; Davis, J. T., submitted for publication in J. Am. Chem. Soc. 1997.

4. “Solid Phase Extraction of Ions Using Molecular Recognition Technology.” Izatt, R. M.; Bradshaw, J. S.; Bruening, R. L.; Tarbet, B. J.; Bruening, M. L., Pure & Appl. Chem. 1995, 67, 1069-1074.

5. “Solvent Extraction Recovery of Byproduct ¹³⁷Cs and ⁹⁰Sr from HNO₃ Solutions-A Technology Review and Assessment.” Schultz, W. W.; Bray, L. A., Sep. Sci. Tech. 1987, 22, 191-214.

6. (a) “Equilibrium Aspects of the Extraction of Cesium Nitrate by Dicyclohexano-21-crown-7, Dibenzo-21-crown-7 and bis-[tert-Alkylbenzo]-21-crown-7 in 1.2-Dichloro-berzene.” Deng, Y.; Sachlben, R. A.; Moyer, B. A., J. Chem. Soc. Faraday Trans. 1995, 91, 4215-4222. (b) “Selective Extraction of Cesium from Acidic Nitrate Solutions with Didodecylnapthalenesulfonic Acid Synergized with Bis(tert-butylbenzo)-21-crown-7.” McDowell, W. J.; Case, G. N.; McDonough, J. A.; Bartsch, R. A., Anal. Chem. 1992, 64, 3013-3017.

7. “Synthesis, Complexation, and Membrane Transport Studies of 1,3-Alternate Calix[4]arene-crown-6 Conformers: A New Class of Cesium Selective Ionophores.” Casnati, A.; Pochini, A.; Ungaro, R.; Ugozzoli, F.; Arnaud, F.; Fanni, S.; Shwing, M.; Egherink, R. J. M.; De Jong, F.; Reinhoudt, D. N., J. Am. Chem. Soc. 1995, 117, 2767-2777.

8. “Molecular Recognition of Necleotides by the Guanidinium Unit at the Surface of Aqueous Micelles and Bilayers. A Comparison of Microscopic and Macroscopic Interfaces.” Onda, M.; Yoshiva, K.; Koyano, H.; Ariga, K.; Kunitake, T., J. Am. Chem. Soc. 1996, 118, 8524-8530.

9. Environmental Radioactivity; Eisenbud, M., Academic Press, London, 1987.

10. (a) Bradshaw, J. S.; Baxter, S. L.; Lamb, J. D.; Izatt, R. M.; Christensen, J. J., J. Am. Chem. Soc. 1981, 103, 1821-1827. (b) Baker, D. S.; Gold. V.; Sghibartz, C. M., J. Chem Soc. Perkin II 1983, 1121-1132. (c) McDowell, W. J.; Case, G. N.; McDonough, J. A.; Bartsch, R. A., Anal. Chem. 1992, 64, 3013-3017. (d) Deng, Y.; Sachleben, R. A.; Moyer, B. A., A. J. Chem. Soc. Faraday Trans. 1995, 91, 4215-4222.

11. For examples of Cs⁺ ionophores, see: (a) Cram, D. J.; Carmack, R. A.; deGrandpre, M. P.; Lein, G. M.; Goldberg, I.; Knobler, C. B.; Maverick, E. F.; Trueblood, K. N., J. Am. Chem. Soc. 1987, 109, 7068-7073. (b) Bryant, J. A.; Ho, S. P.; Knobler, C. B.; Cram, D. J., J. Am. Chem. Soc. 1990, 112, 5837-5843. (c) Krakowiak, K.; Bradshaw, J. S.; Zhu, C.; Hathaway, J. K.; Dalley, N. K.; Izatt, R. M., J. Org. Chem. 1994, 59, 4082-4086.

12. Ungaro, R.; Casnati, A.; Ugozzli, F.; Pochini, A.; Dozol, J. F.; Hill, C.; Rouquette, H., Angew. Chem. Int. Ed. Engl. 1994, 33, 1506-1509.

13. (a) Casnati, A.; Pochini, A.; Ungaro, R.; Ugozzoli, F.; Arnaud; F.; Fanni, S.; Schwiing, M. J.; Egberink, R. J. M.; de Jong, F.; Reinhoudt, D. N., J. Am. Chem. Soc. 1995, 117, 2767-2777. (b) Rudkevich, D. M.; Mercer-Chalmers, J. D.; Verboom, W.; Ungaro, R.; de Jong, F.; Reinhoudt, D. N., J. Am. Chzem. Soc. 1995, 117, 6124-6125.

14. (a) Nechifor, A. M.; Philipse, A. P.; de Jong, F.; van Duynhoven, J. P. M.; Egberink, R. J. M.; Reinhoudt, D. N., Langmuir 1996, 12, 3844-3854. (b) Arnaud-Neu, F.; Asfari, Z.; Souley, B.; Vicens, J., New J. Chem. 1996, 20, 453-463.

15. For examples of self-assemblies constructed using hydrogen bonds, see: (a) Lawrence, D. S.; Jiang, T.; Levitt, M., Chem. Rev. 1995, 95, 2229-2260. (b) Philp, D.; Stoddart, J. F., Angew. Chem. Int. Ed. Engl. 1996, 35, 1154-1196. (c) Zimmerman, S. C.; Duerr, B. F., J. Org. Chem. 1992, 57, 2215-2217. (d) Persico, F.; Wuest, J. D., J. Org. Chem. 1993, 58, 95-99. (e) Branda, N.; Wyler, R.; Rebek, J., Jr. Science 1994, 263, 1267-1268. (f) Yang, J.; Marenda, J. L.; Geib, S. J.; Hamilton, A. D., Tetrahedron Lett., 1994, 35, 3665-3668. (g) Mascal, M.; Hext, N. M.; Warmuth, R.; Moore, M. H.; Turkenburg, J. P., Angew. Chem. Int. Ed. Engl. 1996, 35, 2204-2206.

16. For examples of self-assembled ionophores, see: (a) Schepartz, A.; McDevitt . J. P., J. Am. Chem. Soc. 1989, 111, 5976-5977. (b) Schall, O. F.; Robinson, K.; Atwood, J. L.; Gokel, G. W., J. Am. Chem. Soc. 1993, 115, 5962-5969. (c) Gottarelli, G.; Masiero, S.; Spada, G. P, J. Chem. Soc. Chem. Commun. 1995, 2555-2557.

17. (a) Davis, J. T.; Tiiumala, S.; Jenssen, J. R.; Radler, E.; Fabris, D., J. Org. Chem. 1995, 60, 4167-4176. (b) Tirumala, S.; Davis, J. T., J. Am. Chem. Soc. 1997, in press.

18. The oligonucleotide d(T₄isoG₄T₄) also forms quartets in the presence of Na⁺; Seela, F.; Wei, C.; Melenewski, A., Nucleic Acids Res. 1996, 24, 4940-4945.

19. Cesium-133 NMR has been used to study Cs⁺ coordination by ionophores, see: (a) Mei, E.; Dye, J. L.; Popov, A. I., J. Am. Chem. Soc. 1976, 98, 1619-1620. (b) Mei, E.; Popov, A. I.; Dye, J. L., J. Am. Chem. Soc. 1977, 99, 6532-6536. (c) Kauffmann, E.; Dye, J. L.; Lehn, J. M.; Popov, A. I., J. Am. Chem. Soc. 1980, 102, 2274-2278. (d) Bauer, W., Mag. Res. Chem. 1991, 29, 494-499. (e) Assinus, R.; Böhmer, V.; Harrowfield, J. M.; Oden, M. I.; Richmond, W. R.; Skelton, B. W.; White, A. H., J. Chem. Soc. Dalton Trans. 1993, 2427-2433.

20. Wehrli, F. W., J. Magn. Reson. 1977, 25, 575-580.

21. Bull, T. E.; Forsen, S.; Turner, D. L., J. Chem. Phys. 1979, 70, 3106-3111.

22. (a) Cram, D. J., Angew. Chem. Int. Ed. Engl. 1986, 25, 1039-1057. (b) Cram, D. J., Angew. Chem. Int. Ed. Engl. 1988, 27, 1009-1020.

23. The Cs⁺ selectivity for isoG isopropylidene 1 is remarkable considering that related G tetramers, both in nucleosides and in oligonucleotides, are K⁺ selective: (a) Pinnavaia, T. J.; Marshall, C. L.; Mettler, C. M.; Fisk, C. L.; Miles, T.; Becker, E. D., J. Am. Chem. Soc. 1978, 100, 3625-3627. (b) Hardin, C. C.; Watson, T.; Corregan, M.; Bailey, C., Bioichemistry 1992, 31, 833-841.

24. “Thermodynamic and Kinetic Data for Cation-Macrocycle Interaction.” Izatt, R. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Chri-istensen, J.; Chem. Rev. 1995, 85, 271-339.

25. Seela, F.; Frölich, T., Helv. Chim. Acta, 1994, 77, 399. 

What is claimed is:
 1. An ionophore comprising a plurality of monomers, wherein each monomer is noncovalently bound to another monomer; wherein each monomer is 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isoguanosine; wherein the ionophore is capable of forming a complex with a cation selected from the group consisting of Cs⁺, Ag⁺, Hg³⁰ ², Pb⁺² and Cd⁺².
 2. The ionophore of claim 1, wherein each monomer is bound to another monomer by a hydrogen bond.
 3. The ionophore of claim 1, wherein the ionophore comprises a plurality of identical monomers.
 4. The ionophore of claim 1, wherein the ionophore comprises four monomers.
 5. The ionophore of claim 1, wherein the ionophore comprises eight monomers.
 6. The ionophore of claim 1, wherein the cation is ¹³⁷Cs⁺.
 7. The ionophore of claim 1, wherein the binding affinity of the ionophore for Cs⁺ is greater than the binding affinity of the ionophore for Na⁺ or K⁺.
 8. The ionophore of claim 1, wherein the ionophore is capable of spontaneously assembling in a solution comprising the plurality of monomers.
 9. A complex comprising an ionophore and an ion, wherein the ionophore comprises a plurality of monomers, and each monomer is noncovalently bound to another monomer; wherein each monomer is 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isognosine 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-thio-isoguanosine.
 10. The complex of claim 9, wherein each monomer is bound to another monomer by a hydrogen bond.
 11. The complex of claim 9, wherein the ionophore comprises a plurality of identical monomers.
 12. The complex of claim 9, wherein the ionophore comprises a plurality of identical monomers.
 13. The complex of claim 9, wherein the ionophore comprises four monomers.
 14. The complex of claim 9, wherein the ionophore comprises eight monomers.
 15. The complex of claim 9, wherein the ionophore is capable of forming a complex with a cation.
 16. The complex of claim 15, wherein the cation is Cs⁺, Na⁺, K⁺, Ag⁺, Hg⁺², Pb⁺², or Cd⁺².
 17. The complex of claim 16, wherein the cation is ¹³⁷Cs⁺.
 18. The complex of claim 16, wherein the binding affinity of the ionophore for Cs⁺ is greater than the binding affinity of the ionophore for Na⁺ or K⁺.
 19. The complex of claim 9, wherein the ionophore is capable of spontaneously assembling in a solution comprising the plurality of monomers.
 20. An ionophore comprising a plurality of monomers, wherein each monomer is noncovalently bound to another monomer; wherein each monomer is 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-thio-isoguanosine; wherein the ionophore is capable of forming a complex with a cation selected from the group consisting of Cs⁺, Ag⁺, Hg⁺², Pb⁺² and Cd⁺².
 21. The ionophore of claim 20, wherein each monomer is bound to another monomer by a hydrogen bond.
 22. The ionophore of claim 20, wherein the ionophore comprises a plurality of identical monomers.
 23. The ionophore of claim 20, wherein the ionophore comprises four monomers.
 24. The ionophore of claim 20, wherein the ionophore comprises eight monomers.
 25. The ionophore of claim 20, wherein the cation is ¹³⁷Cs⁺.
 26. The ionophore of claim 20, wherein the binding affinity of the ionophore for Cs⁺ is greater than the binding affinity of the ionophore for Na⁺ or K⁺.
 27. The ionophore of claim 20, wherein the ionophore is capable of spontaneously assembling in a solution comprising the plurality of monomers.
 28. An ionophore comprising a plurality of monomers, wherein each monomer is noncovalently bound to another monomer; wherein each monomer is 2′,3′-Di-O-acetyl-5′-(t-butyl-dimethylsilyl)-isoguanosine; wherein the ionophore is capable of forming a complex with a cation selected from the group consisting of Cs⁺, Ag⁺, Hg⁺², Pb⁺² and Cd⁺².
 29. The ionophore of claim 28, wherein each monomer is bound to another monomer by a hydrogen bond.
 30. The ionophore of claim 28, wherein the ionophore comprises a plurality of identical monomers.
 31. The ionophore of claim 28, wherein the ionophore comprises four monomers.
 32. The ionophore of claim 28, wherein the ionophore comprises eight monomers.
 33. The ionophore of claim 28, wherein the cation is ¹³⁷Cs⁺.
 34. The ionophore of claim 28, wherein the binding affinity of the ionophore for Cs⁺ is greater than the binding affinity of the ionophore for Na⁺ or K⁺.
 35. The ionophore of claim 28, wherein the ionophore is capable of spontaneously assembling in a solution comprising the plurality of monomers.
 36. A method for forming a complex comprising an ionophore and an ion, wherein the ionophore comprises a plurality of monomers, and each monomer is noncovalently bound to another monomer; wherein the method comprises adding the plurality of monomers to a solution containing the ion, thereby forming the complex; wherein each monomer is 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isoguanosine or 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-thio-isoguanosine or 2′,3′-Di-O-acetyl-5′-(t-butyl-dimethylsilyl)-isoguanosine.
 37. A micelle comprising a plurality of ionophores, wherein each ionophore comprises a plurality of monomers, each monomer comprises a hydrophobic moiety, and each monomer is noncovalently bound to another monomer; wherein each monomer has the following structure:

 wherein R is H or —SiR″₃, where R″ is a hydrocarbon (CH₂)_(m) CH₃ or an ester C═O(CH₂)_(n) CH₃, wherein m and n are independently 0-22.
 38. The micelle of claim 37, wherein each monomer is bound to another monomer by a hydrogen bond.
 39. The micelle of claim 37, wherein the ionophore comprises a plurality of identical monomers.
 40. The micelle of claim 37, wherein the ionophore comprises four monomers.
 41. The micelle of claim 37, wherein the ionophore comprises eight monomers.
 42. The micelle of claim 37, wherein the ionophore is capable of forming a complex with a cation.
 43. The micelle of claim 42, wherein the cation is Cs⁺, Na⁺, K⁺, Ag⁺, Hg⁺², Pb⁺², or Cd⁺².
 44. The micelle of claim 43, wherein the cation is ¹³⁷Cs⁺.
 45. The micelle of claim 43, wherein the binding affinity of the ionophore for Cs⁺ is greater than the binding affinity of the ionophore for Na⁺ or K⁺.
 46. The micelle of claim 37, wherein the ionophore is capable of spontaneously assembling in a solution comprising the plurality of monomers.
 47. A method for removing an ion from a solution comprising: (a) adding a plurality of monomers to the solution, thereby forming a complex comprising an ionophore and the ion; wherein the ionophore comprises the plurality of monomers, and each monomer is noncovalently bound to another monomer; and (b) removing the complex from the solution, thereby removing the ion from the solution; wherein each monomer is 2′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-isoguanosine or 5′-(t-butyl-dimethylsilyl)-2′,3′-O-isopropylidene-thio-isoguanosine or 2′,3′-Di-O-acetyl-5′-(t-butyl-dimethylsilyl)-isoguanosine.
 48. A method for removing an ion from an aqueous solution comprising: (a) adding a plurality of monomers to the solution, thereby forming a composition comprising a micelle and the ion, wherein the micelle comprises a plurality of ionophores, each ionophore comprises a plurality of monomers, each monomer comprises a hydrophobic moiety, each monomer is noncovalently bound to another monomer, and the ion is bound to one of the ionophores; and (b) removing the composition from the solution, thereby removing the ion from the solution; wherein each monomer has the following structure:

 wherein R is H or —SiR″₃, where R″ is a hydrocarbon (CH₂)_(m) CH₃ or an ester C═O(CH₂)_(n) CH₃, wherein m and n are independently 0-22.
 49. The method of claim 48, wherein the composition is removed from the solution by ultrafiltration. 